I feel that my travel agent has messed up, but they are telling me not. I'd appreciate a bit of clarification if anyone can offer it...

I booked an S bucket Tango+ fare a little while ago. This week I called to change the route (but same orig/destination), and paid a change fee but no fare difference. My new itinerary shows a K bucket Tango fare .

I suspect my TA has just booked the lowest available fare on the new route, and cashed in the old segments to pay for it - with AC keeping the residual.

I thought that once I had purchased the S fare then I could (subject to availability of course) move to other itineraries for the same route on the same fare with no hassle, even if (as in this case) that fare is now priced a little higher than when I bought it.

In case it matters, the originally purchased S fare and the one that ITA prices for the new route have exactly the same full code, so it isn't a subtlety of different types of S fare being used on the two routes. As far as I can tell, taxes, etc are a smidgen lower for the new route.

Am I missing something, or should I be asking the TA to get me back onto the T+ fare even if they have to pay a further change fee ?